1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to a synchronous rectifying circuit of the soft switching power converters.
2. Description of Related Art
FIG. 1 shows a circuit schematic of a conventional soft switching power converter. The soft switching power converter includes a transformer 10 to provide isolation from input voltage VIN to output voltage VO of the power converter for safety. Two switches 111 and 112 develop a half bridge circuit 11 to switch a resonant tank 12 and the transformer 10. The resonant tank 12 includes an inductor 121 and a capacitor 122. The inductor 121 can be an inductance device or the leakage inductance of a primary winding NP of the transformer 10. The inductance L of the inductor 121 and the capacitance C of the capacitor 122 determine the resonance frequency f0 of the resonant tank 12 as follows equation.
                              f          0                =                  1                      2            ⁢                                                  ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The transformer 10 transfers the energy from the primary winding NP to the secondary windings NS1, NS2 of the transformer 10. Two rectifiers 13, 14 and a capacitor 15 perform the rectifying and filter to the transformer 10 for generating the DC output voltage VO at the output of the power converter. The detail skill of the soft switching power converter can be found in text book ‘Resonant Power Converters’ by Marian K. Kazimierczuk and Dariusz Czarkowski, 1995 by John Wiley & Sons, Inc.
Although the soft switching of the power converter achieves high efficiency and low EMI (electric-magnetic interference) performance, the forward voltage of rectifiers 13 and 14 still causes significant power losses. Using transistors as the synchronous rectifiers is a higher efficiency approach, such as ‘Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter’ by Yang, U.S. Pat. No. 7,173,835. However, the disadvantage of this prior art is additional power consumptions caused by saturable inductors, etc.